Workers in the field of designing rotary anodes for conventional x-ray imaging systems have long recognized the advantages of utilizing graphite in such constructions. However, it soon became evident that in using graphite there also exists the danger that when an anode target layer of tungsten, tungsten alloys, molybdenum and molybdenum alloys is in direct contact with graphite, reactions between the layer and the graphite (during manufacture of the rotary target and/or during the use thereof to generate x-ray beam) lead to the formation of a brittle intermediate carbide layer. The patent literature proposes various anode constructions as solutions to this problem, for example U.S. Pat. Nos. 3,660,053; 3,719,854 and British Patent Nos. 1,173,859; 1,207,648 and 1,247,244.
Another patent (U.S. Pat. No. 3,890,521) expresses concern with the formation of tungsten carbide by reaction between a graphite disc, or carrier, and the tungsten target layer while accepting the in situ formation of a carbide layer of tantalum (or presumably of hafnium, niobium or zirconium). The initial assembly of components consists of a graphite carrier upon which are successively deposited a first layer of iridium, osmium or ruthenium, a second layer of hafnium, niobium, tantalum or zirconium and then a target layer (e.g., tungsten). The desired layer of carbide (e.g., tantalum carbide) forms when, during operation of the x-ray tube, carbon diffuses across the first layer and reacts with the second layer. U.S. Pat. No. 3,710,170 is concerned with thermal stresses introduced in the rotary anode structure because of the difference in thermal expansion coefficients between tantalum carbide (U.S. Pat. No. 3,890,521) and the adjoining structure and between graphite (U.S. Pat. No. 3,710,170) and the adjoining structure. However, in the case of U.S. Pat. No. 3,710,170, as well as in U.S. Pat. No. 3,890,521, certain metal carbide content is deliberately employed as part of the solder material. For example, in U.S. Pat. No. 3,710,170 it is proposed that a molybdenum-molybdenum carbide eutectic be prepared by placing graphite in contact with molybdenum and heating to about 2200.degree. C.
Still another concern is evident in British patent No. 1,383,557 wherein a solder layer of zirconium and/or titanium is employed to join graphite to molybdenum, tantalum or an alloy formed between two or more of tungsten, molybdenum, tantalum and rhenium. A carbide layer is formed between the graphite support and the solder layer. Particular temperature control and initial foil thickness are employed to insure survival of the solder layer.
The great variance in thought in the preceding prior art as to how to best join graphite to refractory metals, particularly tungsten, tungsten alloys, molybdenum and molybdenum alloys shows how complex this problem has remained in the design of rotary anodes for conventional x-ray apparatus.
These varied solutions to the extent they may be viable in conventional x-ray imaging systems, face a much more severe test in connection with the use of graphite members in x-ray tubes used in medical computerized axial tomography (C.A.T.) scanners. For the formation of images, medical C.A.T. scanner typically requires an x-ray beam of about 2 to 8 seconds duration. Such exposure times are much longer than the fractions-of-a-second exposure times typical for conventional x-ray imaging systems. As a result of these increased exposure times, much larger amount of heat (generated as a by-product of the process of x-ray generation in the target region) must be stored and eventually dissipated by the rotating anode.
Graphite, which provides a low mass, high heat storage volume, remains a prime candidate for rotating anode structures of C.A.T. scanner x-ray tubes, particularly when the graphite member functions as a heat sink from which heat is dissipated as radiant energy as disclosed in U.S. Pat. No. 3,710,170 and U.S. Pat. No. Re. 31,568.
One important consideration in the manufacture of a composite anode disc embodying a graphite member is the method by which the graphite is bonded to an adjacent tungsten, tungsten alloy, molybdenum or molybdenum alloy metallic surface. In spite of the favorable view taken of the presence of carbides of tantalum, hafnium, niobium, zirconium and of the eutectic of molybdenum carbide and molybdenum in U.S. Pat. No. 3,710,170 and/or U.S. Pat. No. 3,890,521, workers in the art view with alarm the formation of any layer of tungsten carbide or molybdenum carbide between the graphite member and an adjacent tungsten, tungsten alloy, molybdenum or molybdenum alloy surface to which the graphite must remain bonded. Formation of such a carbide layer is of particular concern, because of the propensity thereof for delamination. Delamination results in a reduction in heat flow from the anode target layer to the adjacent graphite member and loss of structural integrity of the anode which typically rotates at about 10,000 to about 15,000 revolutions per minute.
In x-ray tubes used in C.A.T. scanners, the bulk temperatures during operation of such anode reach about 1200.degree. C.-1300.degree. C. At such temperatures, tungsten, tungsten alloys, molybdenum or molybdenum alloys readily form the undesired metal carbide. Thus, it has been considered particularly important for such rotary anodes to devise a joining procedure and anode structure in which the metallic surface is not permitted to react with the graphite and, even more important, that provision is made in the composite anode structure to prevent reaction from occurring between the metallic surface and the graphite during operation of the C.A.T. scanner x-ray tube.
Three reissue patents (U.S. Pat. Nos. Re. 31,369; 31,560 and 31,568) issued to Thomas M. Devine, Jr., describe a brazing procedure in which a layer of platinum, palladium, rhodium, osmium, ruthenium or platinumchromium alloy is interposed between the metallic surface and the graphite body to which it is to be joined. Although a brazed region develops above and below the interposed layer, this layer itself survives to function as a barrier to carbon diffusion during operation of the x-ray tube. The aforementioned braze materials are characterized by their ability to react with tungsten, tungsten alloys, molybdenum, molybdenum alloys and also with graphite. Because the reaction of the interposed layer with graphite can only proceed at a temperature in excess of the temperatures that are reached by the rotating anode in service, even at the maximum service temperatures an intermediate platinum layer, for example, will act as a diffusion barrier for carbon to prevent the passage thereof through the platinum, where it would be able to form brittle tungsten or molybdenum carbide.
The use of alloys of platinum as an intermediate layer to join graphite to tungsten or tungsten alloy is disclosed in Gebrauchmuster No. 7,112,589 and the use of alloys containing platinum as an intermediate layer to join graphite to tungsten or molybdenum is disclosed in U.S. Pat. No. 3,442,006. In both of these inventions the process for joining requires that the intermediate layer be melted. An intermediate layer of any of the alloys proposed in U.S. Pat. No. 3,442,006 would fail to provide a diffusion barrier to carbon at x-ray anode operating temperatures.
Provided that the brazing in the practice of the aforementioned Devine inventions is accomplished very quickly, formation of the objectionable carbide is avoided. At the typical brazing temperatures employed, the intermediate layer (e.g., platinum) melts and become saturated with carbon. By way of example, liquid platinum can, over a period of time at a temperature just above the eutectic temperature, dissolve up to about 16 atomic percent carbon. When tungsten or molybdenum is in contact with such a high carbon content liquid, carbide will form at the interface. The amount of time available for carbon to dissolve in the liquefied braze layer is, therefore, important and if the assembly being brazed remains at a high temperature for too long a period of time, a thick layer of carbide can form, which could delaminate during cooling or handling. In the case of the use of platinum as the braze layer to affix molybdenum to graphite, a temperature exposure of about 1800.degree. C. for as little as about 5 minutes will result in a layer of molybdenum carbide about 0.003 inch in thickness.
The aforementioned drawback of carbide formation has been addressed in U.S. Pat. Nos. 4,901,338; 4,352,041; 3,579,022 and 3,539,859, U.K. Patent Specification No. 1247244, U.K. Patent Application No. 2084124 A and French Patent Publication No. 2625033 A1 by providing an intermediate layer of rhenium to separate the anode target layer from the underlying graphite anode body. Since adhesion of the intermediate layer to the surface of the graphite anode body is critical, it would be desirable to provide methods of improving adhesion for the intermediate layer to the surface of the graphite anode body, for producing high performance rotary anodes suitable in the increasingly rigorous environment of the C.A.T. scanner x-ray tube.